1. Field of the Invention
The present invention relates to an optical communications system applicable to long-distance communications such as underseas cable communications, etc.
2. Description of the Related Art
Recently, optical communications systems have been widely developed to realize large-capacity and high-speed communications systems. Especially when a large volume of information is to be simultaneously transmitted, an optical wave-length multiplexing system is highly evaluated and is studied for practical use in the near future. In the optical wave-length multiplexing system, an optical signal which carries information and have a plurality of wavelengths is wavelength-multiplexed for transmission. An optical signal of each wave length corresponds to at least one communications channel. In the optical wavelength multiplexing system applicable to the long-distance communications such as underseas cable communications, an optical add-drop system is under development in which an optical signal having a specific wavelength or an optical signal along a specific channel among optical signals wavelength-multiplexed in the communications line is branched to transmit an optical signal along a channel branched to a terminal station, and the optical signal transmitted from the terminal station with the same wavelength as the branched channel is combined again to the optical signal transmitted through the original transmission line for transmission to the terminating station.
FIGS. 1A through 1F show the conventional optical add-drop system and the problem with the system.
FIG. 1A is a block diagram showing the entire configuration of the optical add-drop system. The basic configuration in the optical add-drop system has a terminal station A as a transmitting station for transmitting an optical-wavelength multiplexed optical signal, a terminal station C as a receiving station for receiving a signal from the terminal station A, a branching unit 1100 for branching or combining an optical signal of a specific wavelength in the optical signals from the terminal station A, and a terminal station B for receiving the optical signal branched by the branching unit 1100, and transmitting new information with an optical signal having the same wavelength as the received optical signal. Normally in the underseas cable communications, the branching unit 1100 is mounted underseas to transmit optical signals to, for example, the terminal stations A, B, and C provided in different nations. Typically, the distance between the terminal stations A and C is approximately 3,000 km, and the branching unit 1100 is provided around the central point between these stations. Since the intensity of an optical signal is attenuated when the optical signal is transmitted for a long distance, the transmission lines between the terminal station A and the branching unit 1100, between the terminal station B and the branching unit 1100, and between the terminal station C and the branching unit 1100 have a plurality of optical amplifiers 1101, 1102, and 1103 respectively. FIG. 1A shows the optical amplifiers 1101, 1102, and 1103 apiece for respective transmission lines for a simple illustration, but there are actually much more optical amplifiers for each transmission line. Normally, each of the optical amplifiers 1101, 1102, and 1103 has an automatic output level control circuit (ALC circuit) to keep the output level of each of the optical amplifiers 1101, 1102, and 1103 constant so that the optical signal can be constantly amplified to a specific output level.
FIG. 1A shows the transmission line for one-way communications. Actually, the circuit is designed to establish two-way communications, that is, up-line and down-line communications.
FIGS. 1B through 1F show an optical signal and its problem in each transmission line.
FIG. 1B shows the optical signal at point A in FIG. 1A. In the case shown in FIG. 1B, optical signals having four different wavelengths are wavelength-multiplexed and transmitted from the terminal station A. The mound under each optical signal is called an amplified spontaneous emission (ASE) noise. It is produced when a noise superposed to an optical signal is amplified with the optical signal by an optical amplifier. The characteristics of the operations of the optical communications system depend on the S/N ratio of the optical signal to the ASE.
In the branching unit 1100, the optical signal having a wavelength xcex1 is branched and transmitted to the terminal station B, and an optical signal having the wavelength xcex1 is transmitted from the terminal station B to the terminal station C.
An optical signal having a wavelength other than wavelength xcex1 in the signal (FIG. 1B) transmitted from the terminal station A is not branched by the branching unit 1100, but is transmitted as is to the terminal station C. The terminal station B receives the optical signal having wavelength xcex1 and transmits an optical signal having the same wavelength xcex1. FIG. 1C shows the state at point B of the signal transmitted from the terminal station B and amplified by the optical amplifier 1102. The branching unit 1100 combines the optical signal having wavelength xcex1 transmitted from the terminal station B with the light having wavelength xcex2 through xcex4, and transmits the result to the terminal station C.
FIG. 1D shows the state at point C of the optical signal from the terminal station B which is combined by the branching unit 1100 and amplified by an optical amplifier 1103. FIGS. 1C and 1D show the case where the power level of an optical signal is equal to that of each other when the optical signal from the terminal station B is combined with the optical signal from the terminal station A. In this case, an optical signal having any wavelength indicates the same S/N ratio to the ASE noise as shown in FIG. 1D.
FIG. 1E also shows the state of the optical signal at point B. In this case, the power level of the optical signal from the terminal station B is high. When the power level of the optical signal from the terminal station B is high, the state of the optical signal at point C after being combined by the branching unit 1100 and being amplified by the optical amplifier 1103, becomes as shown in FIG. 1F. Therefore, although the S/N ratio of wavelength xcex1 is high, because the operation characteristics of the optical communications system are based on the lower S/N ratio, when the S/N ratios of the other wavelengths are low, the system is recognized as poor in operation characteristics.
FIGS. 2A, 2B, 3A, and 3B show the operation of the optical amplifier and the S/N ratio.
In this example, the two optical signals having different wavelengths are multiplexed, and an optical signal of a total of 0 dBm power is input to the optical amplifier. The optical amplifier includes an automatic output level control circuit having a gain of 10 dBm and an optical output is limited to 10 dBm. The state of the optical signal at the input terminal is xe2x88x923 dBm each for the power of the optical signals of two wavelengths, a total of 0 dBm as shown in FIG. 2A. FIG. 2B shows the output when such optical signals are input to the optical amplifier. That is, the optical signal of each wavelength is amplified, and the power of each optical signal is +7 dBm with a total power of the output light indicating +10 dBm. On the other hand, the ASE noise is also amplified, and the S/N ratio to the ASE noise of each optical signal is 30 dB. Therefore, the operation characteristic of the optical amplifier indicates the S/N ratio of 30 dB.
FIGS. 3A and 3B show the case where an input optical signal is multiplexed with an optical signal having a different power level. The characteristic of the optical amplifier is the same as that of the optical amplifier shown in FIGS. 2A and 2B. However, as shown in FIG. 3A, a total power of the optical signals having two different wavelengths is 0 dBm with the power level of one optical signal indicating xe2x88x921.5 dBm while the other optical signal indicating xe2x88x924.5 dBm. There is 3 dB difference between the power levels. If such optical signals are input, the output is obtained as shown in FIG. 3B. That is, the higher power level of the optical signal between the two input signals is +8.5 dBm while the lower power level of the optical signal is +5.5 dBm because the optical signal having each wavelength is amplified such that the total power level of the output signals can be the above described value, that is, the output of the optical amplifier is fixed to +10 dBm. At this time, the ASE noise is amplified and the S/N ratios are different between the wavelengths. That is, the S/N ratio of the wavelength indicating the higher power level is an acceptable value while the S/N ratio of the wavelength indicating the lower power level is relatively undesired. Since the operation characteristic of the optical amplifier is evaluated by the undesired S/N ratio, the performance of the optical amplifier is considered to be poor.
In the optical add-drop system as described above by referring to FIG. 1A, a lot of optical amplifiers are inserted between the terminal station and the branching unit. In the branching unit, an independently generated optical signal from the terminal station A is combined with an optical signal from the terminal station B, and amplified by the optical amplifier. The optical signals of respective wavelengths from the terminal stations A and B may not match in power when they are combined because of the transmission distance and the difference in output. Furthermore, the power level of the optical signal may not be controlled just as designed even if the system has been formed by carefully computing the output power and the attenuation of the optical signal in the designing step. In this case, there arises a difference in S/N ratio between the optical signal having a lower power level and the optical signal having a higher power level after the amplification through the optical amplifier as described by referring to FIGS. 2A, 2B, 3A, and 3B. The operation characteristic of the system is evaluated by the S/N ratio of the optical signal having the lower power level, that is, the undesired S/N ratio.
When the power level of the optical signal from a branch station is different from that of the optical signal from the transmitting station, the evaluation is made based on the lower S/N ratio indicating the transmission characteristic of the optical signal, thereby considering the system to be poor in performance.
The present invention aims at providing an optical communications system capable of compensating the difference between the power level of the optical signal from the transmitting station and the optical signal from the branch station, and maintaining a high system performance.
The optical communications system according to the present invention includes a transmitting station for transmitting a wavelength-multiplexed optical signal; a receiving station for receiving the optical signal; a branch station for receiving an optical signal having a specific wavelength in the wavelength-multiplexed optical signals and transmitting the optical signal on the specific wavelength; and a branching unit for branching the optical signal having the specific wavelength from the optical signal transmitted from the transmitting station, transmitting it to the branch station, and combining the optical signal transmitted from the branch station with the optical signal which has the wavelength other than the specific wavelength and has been transmitted from the branch station. The signals are combined with their power levels matching each other.
In an optical communications system including a transmitting station for transmitting a wavelength-multiplexed optical signal; a receiving station for receiving the optical signal; a branch station for receiving an optical signal having a specific wavelength in the wavelength-multiplexed optical signals and transmitting the optical signal on the specific wavelength; and a branching unit for branching the optical signal having the specific wavelength from the optical signal transmitted from the transmitting station, transmitting it to the branch station, combining the optical signal transmitted from the branch station with the optical signal from the transmitting station; and transmitting the result to the receiving station, the branching unit according to the present invention branches the optical signal having the specific wavelength from the optical signal transmitted from the transmitting station, transmits it to the branch station, and combines the optical signal transmitted from the branch station with the optical signal which has the wavelength other than the specific wavelength and has been transmitted from the branch station. The signals are combined with their power levels matching each other.
Otherwise, the terminal station according to another aspect of the present invention includes an optical transmission signal transmitting unit for generating an optical transmission signal modulated using the data to be transmitted; a dummy light generation unit for generating a dummy light different in wavelength from the optical transmission signal; a wavelength multiplexing unit for wavelength-multiplexing the dummy light and the optical transmission signal; and a level adjustment unit for adjusting the output level of the dummy light.
In the method of controlling the optical communications system according to another aspect of the present invention with a system including a first optical terminal station; a second terminal station, a third terminal station; an optical branching unit for connecting the first through third optical terminal station; and an optical amplifier for maintaining an output signal at a constant level between the optical branching unit and the second optical terminal station wherein the branching unit wavelength-multiplexes the optical transmission signals from the first and second terminal stations and transmits the result to the third terminal station, the second optical terminal station controls the optical transmission signal level of an output light from the optical amplifier by transmitting the dummy light different in wavelength from the optical transmission signal and adjusting the level of the dummy light.
Otherwise, in the terminal station in the optical communications system according to the present invention with a system including a first optical terminal station; a second terminal station, a third terminal station; an optical branching unit for connecting the first through third optical terminal station; and an optical amplifier for maintaining an output signal at a constant level between the optical branching unit and the second optical terminal station wherein the branching unit wavelength-multiplexes the optical transmission signals from the first and second terminal stations and transmits the result to the third terminal station, the second optical terminal station includes an optical transmission signal transmitting unit for generating an optical transmission signal modulated using data to be transmitted; a dummy light generation unit for generating a dummy light having a wavelength different in wavelength from the optical transmission signal; a wavelength multiplexing unit for wavelength-multiplexing the dummy light and the optical transmission signal; and a level adjustment unit for adjusting the output level of the dummy light.
In the optical communications system, the terminal station, or the branching unit according to the present invention, when the optical signals in those transmitted from the transmitting station for transmitting wavelength-multiplexed optical signals, but excluding those having a specific wavelength to be transmitted to the branch station are combined by the branching unit with the optical signals having the specific wavelength transmitted from the branch station, the combination can be performed with the power levels of both optical signals matching each other. Thus, the difference in power level between the optical signals after the combination prevents the S/N ratio of the signal at the lower power level from being lowered and the system performance from being deteriorated. That is, the present invention can realize an optical add-drop system capable of applying the system performance at a high level f or a long time.